Novolac-based microcapsules containing isocyanate reagents for self-healing applications

Microcapsules (MCs) containing isocyanate compounds for use as self-healing materials in waterborne polyurethane coatings have been synthesized, in the presence of modified Novolac resins. With modification of Novolac resin, it is succeeded partial or total protection (Benzylation and Acetylation) of its hydroxyl groups. The idea here is to use a protected Novolac resin as the organic substrate for the encapsulation of the less reactive Isophorone isocyanate (IPDI) while the more reactive one, Methylene diphenyl diisocyanate (MDI), is used for the shell formation. Based on that strategy microcapsules of different morphologies and sizes were obtained, depending on the agitation conditions, as revealed using SEM and optical microscopy. Selective extraction was performed to determine the amount of the less reactive isocyanate (IPDI) stored inside the capsules through FTIR-ATR spectroscopy and isocyanate titration as well as the stability of IPDI inside the capsules over time. As determined, microcapsules based on Acetyl-modified Novolac resin encapsulated 96 wt% of IPDI monomer; this amount is about five and ten times higher than that encapsulated in MCs based on by Benzyl-modified Novolac resin or unprotected Novolac resin, respectively. At the same time, MCs based on Acetyl-modified Novolac resin were stable, maintaining approximately 80 % of the initial isocyanate content after two months of storage under inert conditions. Finally, the self-healing ability of the microcapsules was tested by adding selected IPDI-loaded microcapsules in waterborne polyurethane dispersions. It was proven that the Acetyl-modified Novolac-based MCs showed efficient healing behavior, in the absence of any catalyst, on the polyurethanes’ surfaces when scratched artificially.